Conventional gearboxes comprise a main shaft, a counter shaft, a plurality of gearwheel pairs each comprising a primary gearwheel arranged on the counter shaft and a secondary gearwheel rotatably arranged on the main shaft by means of roller bearings. The primary gearwheel and the secondary gearwheel of the respective gearwheel pairs are continuously in meshing engagement with each other. The primary gearwheels on the counter shaft are positioned at a lower level than the secondary gearwheels on the main shaft. The gearbox comprises an oil sump at a bottom portion. The oil sump is filled with gearbox oil up to a level such that substantially all primary gearwheels are continuously in contact with the oil in the oil sump. During operation, the primary gearwheel of the respective gearwheel pairs distributes oil from the oil sump up to its meshing engagement with the secondary gearwheel. The oil provides lubrication and cooling of the meshing engagement of the gearwheels.
A pump pumps continuously oil from the oil sump to the roller bearings of the secondary gearwheels via an oil channel in the main shaft. The oil flow to the roller bearings is dimensioned such that the roller bearings obtain a required lubrication and cooling when they are heavily loaded. During operating situations when the roller bearings are less loaded, the oil flow to the roller bearings is unnecessary large. As a consequence, the pump consumes more energy than necessary during operating situations when they are less loaded. Furthermore, the too large quantity of oil supplied to the roller bearings results in unnecessary rolling losses in the roller bearings. Consequently, the power losses in the gearbox are unnecessary large during operating situations when the roller bearings are less loaded.
The oil sump comprises a substantially constant oil level defining an immersion depth of the respective primary gearwheels into the oil at which a required lubrication and cooling of the meshing engagement of the gearwheel pairs during operating situations when a maximum torque is transmitted through the gearbox. As a consequence, the immersion depth is unnecessarily large when a lesser torque is transmitted through the gearbox and when certain gears are engaged in the gearbox and especially when a direct-gear is engaged in the gearbox. In the latter case, the torque in the gearbox is directly transmitted from an input shaft to the main shaft of the gearbox. Since the torque is not transmitted via anyone of the gearwheel pairs, the oil supply to the meshing engagements between the primary gearwheels and the secondary gearwheels is redundant. The counter shaft obtains a rotary resistance when the primary gearwheels rotates in contact with oil in the oil sump. Consequently, a constant high oil level in the oil sump results in unnecessary power losses in the gearbox due to the rotary resistance when certain gears are engaged in the gearbox and especially when a direct-gear is engaged.
WO 2008/076061 shows a gearbox with a main shaft and a counter shaft supporting a number of gearwheel pairs. During direct-drive operation in the gearbox, the normal oil level in the gearbox is temporarily lowered such that the gearwheels on the counter shaft rotate without contact with the oil in the oil sump. The oil level is raised back to normal level as soon as the direct drive gear is disengaged. An oil gathering tank is arranged in the gear box. The oil gathering tank comprises an inlet arranged in a position such it receives oil splash from the gearwheels on the counter shaft when they come in contact with the oil. The oil flows out from the gathering tank and back to the oil sump via an outlet of the gathering tank regulated by a valve.